In a typical cellular network, also referred to as a wireless communication system, UEs communicate via a radio access network (RAN) to one or more core networks (CNs).
A UE is a device that may access services offered by an operator's core network and services outside operator's network to which the operator's RAN and CN provide access. The UE may be any device, mobile or stationary, enabled to communicate over a radio channel in a communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop, or PC. The UE may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity.
The UE is enabled to communicate wirelessly in the network. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between the UE and a server via a RAN and possibly one or more core networks, comprised within the cellular network.
The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a network node in the form of a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called evolved NodeB (eNB), NodeB, B node or base station. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the UE within range of the base stations.
Standardised by the third Generation Partnership Project (3GPP), High Speed Downlink Packet Access (HSPA) supports the provision of voice services in combination with mobile broadband data services. HSPA comprises High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and HSPA+. HSDPA allows networks based on the Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. In HSDPA, a new transport layer channel, High Speed Downlink Shared Channel (HS-DSCH), has been added to the UMTS release 5 and further specification. It is implemented by introducing three new physical layer channels: High-Speed Shared Control Channel (HS-SCCH), Uplink High-Speed Dedicated Physical Control Channel (HS-DPCCH) and High-Speed Physical Downlink Shared Channel (HS-PDSCH). The HS-SCCH informs the UE that data will be sent on the HS-DSCH, 2 slots ahead. The HS-DPCCH carries acknowledgment information and a current Channel Quality Indicator (CQI) value of the UE. This CQI value is then used by the base station to calculate how much data to send to the UE on the next transmission. The HS-PDSCH is the channel mapped to the above HS-DSCH transport channel that carries actual user data. HSPA may recover fast from errors by using Hybrid Automatic Repeat reQuest (HARQ). HARQ is a technique that enables faster recovery from errors in communications networks by storing corrupted packets in the receiving device rather than discarding them. Even if retransmitted packets have errors, a good packet may be derived from the combination of bad ones.
Multiple Input Multiple Output (MIMO) refers to any communications system with multiple antennas at the transmitter and/or the receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter (Tx), multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver (Rx), multiple antennas may be used for receiver combining which provides diversity and combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the minimum number of receiver antennas required is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, may support up to four data streams. In general, MIMO may be n×n MIMO, where n is the number of antennas and is positive integer. For example 2×2 MIMO, 8×8 MIMO, 16×16 MIMO etc.
Several new features are added for the long term High Speed Packet Access (HSPA) evolution in order to meet the requirements set by the International Mobile Telecommunications Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch MIMO, i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beam forming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high Signal to Noise Ratio (SNR) UEs and improves the coverage for low SNR UEs.
Channel feedback information enables a scheduler to decide which UE should be served in parallel. The UE is configured to send three types of channel feedback information: Channel Quality Indicator (CQI), Rank Indicator (RI) (which indicates the number of transport blocks preferred) and Pre-coding Matric Indicator (PMI), which is also known as Precoding Control Indicator (PCI). CQI is an important part of channel information feedback. The CQI provides the base station with information about link adaptation parameters which the UE supports at the time. The CQI is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams. RI is the UE recommendation for the number of layers or transport blocks, i.e. streams to be used in spatial multiplexing. RI is only reported when the UE operates in MIMO mode with spatial multiplexing. The RI may have the values 1 or 2 in a 2×2 MIMO configuration and it may have the values from 1 and up to 4 in a 4×4 MIMO configuration. The RI is associated with a CQI report. This means that the CQI is calculated assuming a particular RI value. The RI typically varies more slowly than the CQI. PMI provides information about a preferred pre-coding matrix in a codebook based pre-coding. PMI is only reported when the UE operates in MIMO. The number of pre-coding matrices in the codebook is dependent on the number of antenna ports on the base station. For example, four antenna ports enables up to 64 matrices dependent on the RI and the UE capability. PCI indicates a specific pre-coding vector that is applied to the transmit signal at the base station.
Introduction of four branch MIMO will require a new feedback channel structure to send the CQI/PCI information to the base station. To reduce the signalling overhead at downlink and uplink, it was recommended to use two code words for four branch MIMO. For designing uplink signalling channel, i.e. HS-DPCCH, it was agreed to use a similar structure as that of 2 Antenna MIMO, described in 3GPP Release-7 (see 3GPP TS 25.214). When reporting CQI, RI and PCI, this channel state information (CSI) may be reported in two reporting intervals. This structure is attractive in terms that it requires minimal standards change. The performance with this structure is very close to that of ideal reporting. In general, the base station needs to wait for two reporting intervals to schedule the UE for data transmission. If the reporting period is configured to a high value, say for example 8 msec, the base station needs to wait 16 msec to schedule the UE. For a high speed UE, this introduces delay and the performance degradation is very severe.
An overview of Channel Quality Reporting and Node B Procedures for Two Branch (2×2) MIMO (Release 7 MIMO) will now be described. FIG. 1 shows the messages exchanged between base station and the UE during a typical data call set up.
The method comprises the following steps, which steps may be performed in any suitable order:
Step 101: The Common Pilot Indicator Channel (CPICH) is a downlink channel broadcast by the base station with constant power and of a known bit sequence.
Step 102: From the CPICH in step 101, the UE estimates the channel conditions and computes the CQI and the PCI, which is the precoding information bits selected in the subset of the codebook corresponding to the rank information. For two antennas, the CQI is computed as: CQI=15×CQI1+CQI2+31, when 2 transport blocks are preferred by the UE; and is computed as: CQI=CQIs when 1 transport block is preferred by the UE, where the CQI is the channel quality per individual layer, CQIs is the CQI value in the case of Rank=1 (i.e., one transport block is preferred), CQI1 and CQI2 are the individual CQI values for each stream in the case of Rank=2 (i.e., two transports blocks are preferred). It can be observed that if the computed CQI value is less than 31, the rank information is 1 (i.e., one transport blocks is preferred), otherwise the rank information is 2 (i.e., two transports blocks are preferred).
Step 103: The information computed in step 102, i.e. the CQI and PCI, along with a HARQ ACK/NAK is reported (i.e., transmitted) to the base station using the HS-DPCCH. The periodicity of HS-DPPCH is one subframe (e.g. 2 msec). The structure of the HS-DPCCH is shown in FIG. 2a and FIG. 2b. In FIG. 2a, an example of how the PCI and the CQI are located in the structure is shown. The HS-DPCCH sub-frame structure comprises one slot for HARQ-ACK transmissions and two slots for CQI/PCI transmissions. Even though the text or the drawings refer to a HARQ ACK, it is appreciated that this may also be a HARQ NACK.
The HS-DPCCH sub-frame structure in FIG. 2a for the TTI=2 ms comprises a HARQ ACK or NACK which notifies the base station that the UE has received correct downlink data or not. The field defines like this: 1-NACK, 0-ACK. The CQI reflects the PCI based on CPICH strength. Each sub-frame comprises a HARQ ACK, two CQI-fields and one PCI field. In other words, every sub-frame comprises the same fields.
The HS-DPCCH in 3GPP Rel-5 to Rel-9 is based on a 1×SF256 solution (see TS 25.212). The structure of the HS-DPCCH is shown in FIG. 2b. The HS-DPCCH sub-frame structure consists of 1 slot for HARQ-ACK transmissions and 2 slots for CQI/PCI transmissions. This structure should also be used for 4-branch MIMO.
HARQ Details: For 3GPP Rel-7 MIMO (3GPP TS 25.214) the HARQ-ACK codebook comprises 6 codewords plus PRE/POST.
CQI/PCI Details: In 3GPP Rel-7 (3GPP TS 25.214) there are 5 or 2×4 bits allocated for describing the CQI depending on the CQI type. There are 30 or 15 CQI values per stream for rank1 and rank2, respectively, and RI (i.e., rank) is implicitly signalled via the CQI. Furthermore CQIs for each stream are signalled independent of each other. In addition to CQI bits there are 2 bits allocated for signalling the preferred pre-coding information. The 7 (or 10) information bits are then encoded into 20 channel bits that are transmitted during the second and third slot.
Step 104: Once the base station receives the CQI, PCI and HARQ ACK, it allocates the required channelization codes, modulation and coding, precoding channel index to the UE after scheduling.
Step 105: Information about the required channelization codes, modulation and coding, precoding channel index from step 104 is transmitted to the UE using the HS-SCCH.
Step 106: The UE detects the HS-SCCH.
Step 107: Once the UE has detected the HS-SCCH, the downlink transmission starts through data traffic channel using the HS-PDSCH.
In general, HS-DPCCH design depends on many factors like number of codewords supported, number of HARQ processes, precoding codebook etc. Four branch MIMO should support two codeword and two HARQ processes.
The current HSDPA system (3GPP Release 7-10) supports 1 or 2 transmit antennas at the base station. For these systems, from channel sounding, the UE measures the channel and provides in one subframe a channel status report (CSR) that contains channel state information (CSI) (e.g., a CQI). A sub frame may be defined as for example one Transmission Time Interval (TTI) which may be e.g. 1 ms or 2 ms. Typically, a CSR consists of the CQI, which indicates the RI, and the PCI. The UE sends this report periodically for every subframe, i.e. for every TTI to the base station. Once the base station receives this report it grants the Modulation and Coding Scheme (MCS), number of codes, rank and the PCI to each specific UE based on the scheduler metric. Based on this information, the base station may optimize the downlink throughput for each TTI.